In response to the recent development of various optical recording media, optical pickup devices that can carry out recording and reproducing using two alternative types of optical recording media have been known. For example, devices that record or reproduce information with either a DVD (Digital Versatile Disk) or a CD (Compact Disk including CD-ROM, CD-R, CD-RW) have been practically used. Furthermore, the DVD, in order to improve the recording density, is designed to use visible light with a wavelength of approximately 658 nm. In contrast, because there are also optical recording media that do not have any sensitivity to light in the visible light region, near-infrared light with a wavelength of approximately 784 nm is used for the CD. Further, in these two optical recording media, it is necessary to differentiate the numerical apertures (NA) due to the differences in the characteristics of the two optical recording media. Additionally, the substrate thickness, that is, the geometric thickness of a protective layer formed with PC (polycarbonate), of each of the two different optical recording media is standardized to a different thickness. For example, the substrate thickness of the DVD is 0.6 mm and the substrate thickness of the CD is 1.2 mm. In addition, a semiconductor laser with a short wavelength (for example, that emits a laser beam with a wavelength of 408 nm) using a GaN substrate has been put into practical use, and in response to the demand for increasing recording capacity, AODs (Advanced Optical Disks), also known as HD-DVDs, that provide approximately 20 GB of data storage on a single layer of a single side of an optical disk by using this short wavelength light is about to be put into practical use. Further, a Blu-ray Disc (hereafter, referred to as ‘BD’) where a light with a short wavelength is used as an irradiation light similar to the AOD is almost ready to be put into practical use.
In the standards for AODs, the numerical aperture and the substrate thickness are standardized to the same values as those of DVDs, specifically a numerical aperture (NA) of 0.65 and a substrate thickness of 0.6 mm. In contrast, in the standards for BDs (Blu-ray disk systems), the numerical aperture (NA) and the substrate thickness are standardized to be completely different values from the values for DVDs and CDs. Specifically, for BDs, the standard numerical aperture (NA) is 0.85 and the standard substrate thickness is 0.1 mm.
Therefore, an optical pickup device wherein any of three optical recording media (namely, an AOD, DVD and CD, or a BD, DVD and CD) can be used, has also been progressing.
As described above, with these optical recording media, because the standardized wavelengths and substrate thicknesses differ from one another depending upon the type of optical recording medium being used, the spherical aberration generated by the substrates differs based on differences in thicknesses of the substrates (protective layers). Therefore, in these optical pickup devices, because it is necessary to optimize the spherical aberration relative to the light beams of various wavelengths in order to assure a proper focus onto the different recording media for recording or reproducing information, it is necessary to devise a lens configuration that has a different light convergence or divergence effect on each of the optical recording media for the objective lens for optical recording media mounted in these devices.
Applicants of the present invention have already suggested various objective lenses for optical recording media in the specifications of Japanese Laid-Open Patent Applications 2005-190620, 2005-158213, 2005-093030, 2005-149626 and 2005-100586. In the objective lenses for optical recording media of the Japanese applications listed above, light beams of different wavelengths are focused on the recording medium of each of the CD, the DVD, and the AOD (or the BD). This is achieved, for example, using an objective optical system for optical recording media that includes a diffractive surface, which has wavelength selectivity, and an objective lens in order to achieve optimization of correction of spherical aberrations generated by differences in the thicknesses of the substrates (protective layers) of the optical recording media by using the difference in the wavelengths of the operating light beams for the CD, DVD, and AOD (or BD).
As described above, because AODs and BDs are approaching practical use, there is a demand to be able to record and reproduce information using four types of optical recording media, that is, using AODs and BDs, in addition to CDs and DVDs, as the optical recording media using a single objective lens.
However, as described above, light beams with the same wavelength, or very nearly the same wavelength, for example, 408 nm or very nearly 408 nm, are used for both AODs and BDs, and according to the teachings of the Japanese applications listed above, where the light convergence or divergence effects are changed based on differences in wavelengths of the light beams being used, the use of the same wavelength, or very nearly the same wavelength, does not support using both a BD and an AOD with a single objective lens.
Therefore, it is necessary to adopt new concepts in order to realize an objective optical system for optical recording media that can be used for at least both an AOD and a BD.